42 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, 125 



taxonomic characters with which to distinguish the species of micro- 

 filariae developing in the flies. Since 5". ochraceum, metallicum, and 

 calUdum not only attack man, but are also avid feeders on horses and 

 cattle (table 9), it is quite possible that flies found infected in nature 

 may have acquired their infection from the latter animals. The more 

 zoophilic tendencies of 5". metallicum and callidum (table 10) may also 

 explain the higher natural infection rate in these species, reported by 

 both De Leon and Gibson. 



Assuming that the species of Onchocerca infecting humans is the 

 same in both hemispheres, the difference in the length of the develop- 

 mental cycle of O. volvulus in the simuliid vector, as reported by 

 Blacklock (1926b) and Wanson, Henrard, and Peel (1946) in 

 Africa, and by Gibson (i95ia,b) in Guatemala, may be due to the 

 species of black fly involved or to the effect of environmental factors. 



In an attempt to determine whether temperature affects the duration 

 of the developmental stages in the flies, an experiment was conducted 

 by our laboratory in a hot coastal region. Siinulium haematopotum, 

 known to ingest microfilariae and support their subsequent develop- 

 ment experimentally, were collected while they were biting several 

 infected individuals. The flies were maintained in tubes in the same 

 area where they had been captured, and were fed on sugar water. Of 

 295 flies examined upon death, 10 were infected. Eight contained 

 "sausage" forms from 3 to 5 days after their infection, one still con- 

 tained microfilariae 2 days after infection, and one had what appeared 

 to be preinfective larvae 5 days after infection. Biopsies taken from 

 numerous horses and cattle in the region all resulted negatively, thus 

 indicating that the fly infections resulted from feeding on the infected 

 human subjects. Although the number of infected flies examined was 

 small, the results indicate the possibility that high temperature may 

 hasten the development of Onchocerca volvulus in the flies. It was 

 desired to repeat these studies with 5". ochraceum and metallicum, in- 

 fecting them in the highland region of San Pedro Yepocapa where 

 onchocerciasis is endemic, and taking them immediately thereafter to 

 the coastal area for observation of the developmental stages. How- 

 ever, lack of time and other factors precluded the possibility of execut- 

 ing these plans. 



Gibson and Dalmat (1952) found wild-caught 5". exiguum and S. 

 haematopotum naturally infected, but an investigation of the skin 

 biopsies of man, cattle, and horses in the region of capture of the 

 former species proved that its natural infection must be of bovine 

 origin (table 7). The authors were able to infect 5". exiguum, haema- 

 topotum, and veracruzanum by feeding them on subjects infected 



